Energy beams are often used in the shaping of workpieces in industrial fabrication processes. For example, electron beam machining has been used in the fabrication of small intricately shaped parts. With the development of the laser it has become the most widely used energy beam device in such fabrication processes. In the electronics industry, semiconductor monolithic integrated circuits (ICs) are conventionally trimmed with focused laser beams on a mass production basis. As indicated in U.S. Pat. No. 3,584,183 a pulsed laser is used to trim the diodes that form an integrated circuit matrix to generate a desired encoding pattern. In application Ser. No. 830,979 filed Sept. 6, 1977, Robert F. Johnson, and assigned to the assignee of the present invention, laser trimming of active IC elements is described. In application Ser. No. 860,927 filed Dec. 15, 1977 by Robert A Cometta, and assigned to the assignee of the present invention, the application of a pulsed laser to a step and repeat IC wafer production process is shown.
Typically the laser trimming processes employ energy beams that are not visible and therefore aiming such beams becomes a problem. Typically a visible aiming light beam is transmitted coaxially with the invisible working beam and is normally left on while the invisible beam is not energized. The visible spot will then define the point of impingement of the invisible beam when it is turned on. Such aiming requires careful alignment of the two beams and visual observation or some other means for monitoring the visible beam. Alternatively the working beam can be directed to an unused substrate portion and its effect rendered visible on the substrate. One such means is to locate a target area on the subject substrate and to direct the working beam to a reference point. The area to be worked is related by precisely known coordinates and once the target area is impacted, the working area can be directed precisely.
As described in application Ser. No. 860,927 filed Dec. 15, 1977, now U.S. Pat. No. 4,182,024, an IC wafer is located in a step and repeat machine which indexes the semiconductor IC wafer to successively bring each one of the plurality of IC devices located thereon under the influence of the trimming laser. Once a wafer is precision oriented and located in such a machine, the step and repeat function will precisely index the wafer, one circuit at a time, until each one of the wafer is treated. While the stepping operation is very good, it is not perfect and small cumulative error in indexing occurs. Also, if the wafer has a rotational misalignment, a cumulative error can result in a direction normal to the direction of the stepping operation. Thus while the first circuit is precisely aligned with the aimed laser, subsequently treated circuits will be less precisely aligned. Where the cumulative error in either direction exceeds a tolerable limit, the laser must be re-aimed before proceeding. It would be desirable to make this operation automatic. Thus it would be desirable to be able to detect laser misalignment.